Courses Grad Courses or Senior Courses?

I'll be entering my last year for my BS degree in physics this fall. I have Solid State lecture and Solid State Lab lined up for my physics courses. My friends are taking graduate level Quantum Mechanics.

My reasoning is "I already know what QM is, but not SS, and I have to take QM anyway in Grad school."

Their reasoning is "We like QM."

Is there any advantage to taking grad level courses as an undergrad vs. expanding my undergrad knowledge library?

Especially if we consider that it will be harder for me to get a good grade in grad level QM than undergrad SS.

I've already taken Statistical Mechanics, so I have a small taste of SS. I'm mainly doing it to boost my GPA a bit. The Lab is required, though. Should I switch lectures?

Well, it might get you out of a course or two in grad school - not every school requires that you take every course, just that you can pass the qualifying exams. And being able to pass graduate level courses will help you if your GPA isn't great. But if you're trying to boost your GPA, graduate level quantum mechanics is not likely to help.

Only two reasons I could see for taking SS are
-Possibly going in a ss-related field (which, honestly, most physicists do...)
-Not being confident that you'll do well in grad QM (while not taking any grad courses may have the admissions committee wondering whether you can handle it, doing badly in a grad course confirms that you can't)

Are you going to get to take solid state courses in grad school? If not, and you like the idea of learning it, then take it now. Primarily you should think about learning things you are interesting in, not pushing up your GPA.

I think you should do SS, since that is an important area of research to consider. The graduate QM course is best after you have had some time to soak up QM from a different angle e.g. SS, and after you have gained more mathematical maturity.

I think your friends are trying to rush things, but they will probably have to retake QM as grad students to prepare for the qualifying exams. Also, professors care more about your research potential then your coursework potential.

SS would be a great course to let you see some practical implications of QM. I remember distinctly a friend complaining that he didn't see any real implications of QM beyond curiosity (we're engineering majors). I was quick to point out quantum tunneling as a major problem in current-generation CMOS devices as well as its use in floating-gate transistors.